SACRAMENTO — A state judge ruled Tuesday that Gov. Arnold Schwarzenegger’s plan to transfer inmates out of state to relieve severe prison overcrowding is illegal.

The governor said he would immediately appeal, citing the potential for some convicts to be released early.

“Today’s disappointing ruling is a threat to public safety,” Schwarzenegger said in a statement. “I will not release dangerous criminals to relieve overcrowding. The transfer of inmates is imperative to relieve the pressure on our overburdened prison system.”

Schwarzenegger signed an emergency declaration in October ordering the corrections department to send thousands of inmates to private prisons in other states. Two employee unions, including the one representing guards, filed a lawsuit saying the declaration violated state law.

Sacramento County Superior Court Judge Gail Ohanesian agreed with the unions and invalidated California’s contracts with the other states.

“Prison overcrowding in California is a crisis creating conditions of extreme peril,” Ohanesian wrote in her five-page order. Nonetheless, “this is not the type of circumstance generally covered by the Emergency Services Act.”

The act is intended to let the state step in when local governments are overwhelmed and cannot be used to respond to problems that originated at the state level, the judge said.

“The control of state prisons is exclusively within the purview of state government and not local government,” Ohanesian said in her ruling. “The intent of the Emergency Services Act is not to give the governor extraordinary powers to act without legislative approval in matters such as this that are ordinarily and entirely within the control of the state government.”

The California Correctional Peace Officers Association and the Service Employees International Union had challenged Schwarzenegger’s prisoner transfers for violating the emergency act and a provision in the state Constitution. That provision prohibits using private companies for jobs usually performed by state workers.

The judge agreed with the unions on that point, as well.

Tuesday’s ruling was the latest twist in a tangled effort to solve the many problems with California’s prison system.

Lawsuits have placed many of its operations under federal oversight, including inmate health care, mental health, officer discipline and juvenile justice.

Overcrowding is among the most pressing issues. The state’s 33 prisons were designed for roughly 100,000 inmates but now hold more than 174,000.

Makeshift dormitories have been created in gymnasiums and hallways. Corrections officials say the shoulder-to-shoulder conditions already have led to violence and are creating a tinderbox ripe for a large-scale riot.

A federal judge has ordered California to solve the crowding problem by June or face the possibility that the inmate population will be capped and some prisoners released early.

Schwarzenegger proposed the out-of-state transfers as one part of the solution. He also has asked lawmakers to review California’s sentencing laws and consider an $11 billion prison and county jail building program.

Corrections officials had hoped that 2,260 inmates would volunteer during the first round of transfers, but only 460 did.

Earlier this month, Corrections Secretary James Tilton said transfers would be forced. He wanted to move up to 5,000 inmates to private prisons in Arizona, Indiana, Mississippi, Oklahoma and Tennessee.

The unions sued in October after the state entered its first contracts.

Schwarzenegger had argued that the contracts were so urgently needed that it was enough to violate the state’s usual civil service laws. But Ohanesian ruled that the urgency was not created by a lack of prison employees, but rather by a lack of prisons.

“The services which are the subject of the private contracts are available within the civil service,” she wrote.

The government’s argument that there is a five-year backlog in hiring enough prison guards isn’t enough to overturn the prohibition in the state Constitution, she said.

Ohanesian delayed her ruling from taking effect for 10 days to allow time for the governor to appeal.